In general, cognitive radio (CR) is a form of wireless communications wherein either a wireless CR network or a CR node changes its transmission and/or reception parameters in order to communicate efficiently and avoid interference from or interfering with licensed and/or unlicensed users. Therefore, an important feature in CR is the ability to detect the presence of licensed and/or unlicensed transmissions. This is commonly referred to as spectrum sensing.
Spectrum sensing usually involves energy detection within a frequency band of interest and may be achieved using a band-pass filter with a pass-band spanning the frequency band of interest, a received energy measuring device, an accumulator to accumulate the received energy over a desired observation interval, and a decision device to compare the accumulated received energy with a threshold. If the accumulated received energy is greater than the threshold, then transmissions may be deemed to be present in the frequency band of interest. Spectrum sensing may be performed at CR nodes and information arising from the spectrum sensing may be used by the CR nodes to alter their transmission and/or reception parameters.
Recent proposals have been made to perform cooperative spectrum sensing, wherein multiple CR nodes may perform spectrum sensing and then the multiple CR nodes may share the results of the spectrum sensing to help improve overall spectrum sensing performance. The results of the spectrum sensing may be provided to a combining node that may aggregate the cooperating spectrum sensing results and provide the combined cooperative spectrum sensing information to the multiple CR nodes.
Cooperative spectrum sensing may be able to help improve the performance of certain CR nodes that may be prevented from properly detecting the spectrum due to their location. For example, a CR node may be positioned behind a large object or body, such as a large building, a mountain, a large stand of trees, and so forth, which may prevent the CR node from detecting a licensed user located on a far side of the large object. In such a situation, combined cooperative spectrum sensing information from other CR nodes may help the CR node properly adjust its transmission and/or reception parameters.
However, the cooperative spectrum sensing proposals heretofore have been synchronized in nature, wherein the CR nodes all perform the cooperative spectrum sensing at substantially the same time and provide the results of the cooperative spectrum sensing to the combining node. The combining node may then aggregate the cooperative spectrum sensing results based on an assumption that the individual results are based on synchronized observations.
FIG. 1 illustrates spectrum sensing activity of a number of CR nodes performing cooperative spectrum sensing in a synchronized fashion. A first trace 105 displays spectrum sensing activity by a first CR node, a second trace 110 displays spectrum sensing activity by a second CR node, and a third trace 115 displays spectrum sensing activity by a K-th CR node. Operating in synchrony, the first through K-th CR nodes begin and stop spectrum sensing at substantially the same time, shown as shaded boxes 106, 111, and 116. Then, at a time TS, the first through K-th CR nodes may transmit results of their respective spectrum sensing to a combining node over a common control channel, for example. The combining node may at time T receive the results of the spectrum sensing from the first through K-th CR nodes and compute a combined cooperative spectrum sensing information. The combined cooperative spectrum sensing information computed by the combining node may then be provided back to the first through K-th CR nodes.